Explosions at Boston Marathon

In this image from video provided by WBZ TV, spectators and runners run from what was described as twin explosions that shook the finish line of the Boston Marathon, Monday, April 15, 2013, in Boston. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/WBZTV) MANDATORY CREDIT

Medical workers wheel the injured across the finish line during the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

Emergency workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

Medical workers aid an injured man at the finish line of the 2013 Boston Marathon following an explosion there Monday, April 15, 2013 in Boston. Two explosions shattered the euphoria at the finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

Medical workers and authorities work on the scene near the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/WCVB-TV/ABC) MANDATORY CREDIT

In this photo provided by The Daily Free Press and Kenshin Okubo, people react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

A firefighter tends to an injured man following an explosion near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Michael Dwyer)

Boston police on bicycles patrol on Commonwealth Avenue following the explosions at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded at the Boston Marathon finish line Monday killing two people and injuring dozens. (AP Photo/Michael Dwyer)

This photo provided by Bruce Mendelsohn shows the scene after two explosions occurred during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/ Bruce Mendelsohn) MANDATORY CREDIT

Medical workers aid an injured woman at the finish line of the 2013 Boston Marathon following two explosions there, Monday, April 15, 2013 in Boston. Two bombs exploded near the finish of the Boston Marathon on Monday, killing at least two people, injuring at least 23 others and sending authorities rushing to aid wounded spectators. (AP Photo/Charles Krupa)

Women react as they walk from the area where there was an explosion after the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Josh Reynolds)

In this photo provided by The Daily Free Press and Kenshin Okubo, people assist an injured after an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

One of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon is investigated by two people in protective suits in the wake of two blasts in Boston Monday, April 15, 2013. (AP Photo/Elise Amendola)

An unidentified Boston Marathon runner, center, is reunited with loved ones near Copley Square following an explosion in Boston Monday, April 15, 2013. (AP Photo/Winslow Townson)

Debris is spread along the sidewalk of one of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon in the wake of two blasts in Boston Monday, April 15, 2013. (AP Photo/Elise Amendola)

Firefighters tend to a man following an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded at the Boston Marathon finish line Monday killing at least two people injuring dozens. (AP Photo/Michael Dwyer)

Justine Franco of Montpelier, Vt., holds up a sign near Copley Square in Boston looking for her missing friend, April, who was running in her first Boston Marathon Monday, April 15, 2013. Two bombs exploded near the finish line of the marathon on Monday, killing at least two people and injuring at least 23 others. (AP Photo/Winslow Townson)